Cast base alloys used in turbine engine components can be coated with MCrAlY type overlay coatings that typically contain about 8-12% aluminum. These coatings extend the life of the components that they are applied to.
During manufacturing rework or aftermarket repair, coatings are removed using mineral acids. Typically 70-100 v/o hydrochloric acid is used to remove MCrAlY type coatings, which acid leaches preferentially the aluminum in coatings containing relatively elevated levels of aluminum, but does not attack the base alloy which contains much lower levels of aluminum. On a significant number of coating and base alloy systems, the mineral acids used preferentially attack the coating without significant chemical attack or corrosion of the base alloys. The intended result is that the coating is removed without damaging the part.
In addition, the use of hydrochloric acid gives rise to concerns over workplace and external emissions for which emissions and permitting standards have been developed. Accordingly, it would be desirable to be able to provide a coating strip process to remove MCrAlY type coatings while eliminating the use of chloride-based stripping compositions, while achieving at least the same efficacy and efficiency as that achieved with chloride-based stripping compositions and methods.
Improvements in the methods of stripping of superalloys are therefore needed in the art.